Faulty disk, SCSI-errors is something that can be ruled out for sure: "Disks" used by this VM come from a NetApp system providing storage for 200+ virtual machines and none of them is experiencing these problems.

re-installing guest OS from scratch - errors remain

with or without VMware tools installed - errors remain

Set everything up under FreeBSD 9.x instead of FreeBSD 10 - same story

these errors are definitely not related to "heavy io" - the machine in question is acting as a DHCP and caching DNS-server

errors appear repeatedly but not related to a specific time of the day

Even worse: if these errors appear several times in a row the VM completely crashes and has to be "power-cycled" manually

what is the SAS/SCSI/SATA virtual adapter you are using and have you tried changing it around? Think LSI Logic SAS or Parallel switch could help, maybe even trying a SATA controller for your disk storage could help.

Are you sure it's actually fixed ? I'm running 10.1 (kernel & System dating April 15) and the problem is still there, i.e. I'm still getting These Errors, to the log - only the system doesn't crash any more.

I was getting more and more of these errors on my FreeBSD VM as I added more VMs to the ESX. I've now added a memory reservation to the FreeBSD VM (Settings -> Resources -> Memory -> Reservation 3000MB) and it seems to have vastly improved (not seen any in the last 3 days). Guess it might have just been due to too much memory over subscription on the ESX impacting the performance of FreeBSD.